1. Field of the Invention
The present invention relates to an improvement of an unsaturated vapor pressure type high pressure sodium lamp which substantially whole components filled within an arc tube are evaporated during the lighting thereof.
2. Description of the Related Art
Generally a high pressure sodium lamp has an arc tube and an outer tube installing the arc tube therein. A metallic cap having an electrode is inserted into both end openings of a translucent alumina tube as the arc tube and is closely sealed, and sodium, mercury and xenon gas is filled within the arc tube.
The lamp characteristic of the high pressure sodium lamp principally depends on the filling amount of sodium and mercury, the filling ratio of them and a position where surplus sodium and mercury stay during the lighting of the lamp, that is, the temperature of the coolest position in the arc tube. If sodium reacts with the component material of the arc tube, that is, if sodium is consumed the ratio of sodium and mercury is varied. Consequently, the characteristic of the lamp varied, especially, the voltage of the lamp increases to turn off it.
The conventional high pressure sodium lamp is a called saturated vapor pressure type high pressure sodium in which the excess amount of sodium extremely larger than the amount of sodium that vapors during the lighting is filled so as to compensate the consumption of the sodium. There is a little change in the characteristic of the high pressure sodium lamp, even if sodium is consumed during the lighting. However, with respect to the heat influence from the inside and outside, a little change of the temperature of the coolest position causes the extreme change of its lamp characteristic. Accordingly, the conventional high pressure sodium lamp has a large temperature dependency.
On the other hand, in a case where the whole amount of filled sodium and mercury have vapored during the lighting, since the filled materials for vapor do not exist any longer, the vapor pressure no longer increases. Accordingly, even if the temperature of the coolest position is changed, its lamp characteristic is almost never changed. The lamp having such an excellent characteristic is a well-known unsaturated vapor pressure type high pressure sodium lamp.
However, the arc tube of the unsaturated vapor pressure type high pressure sodium lamp is filled with sodium whose amount is only for vaporing during the lighting. Accordingly, if sodium reacts with the component material of the arc tube, the consumption of the sodium appears as the change of its lamp characteristic. Therefore, in order to accomplish the unsaturated vapor pressure type high pressure sodium lamp, sodium have to be prevented from the reaction with the component material of the arc tube. Consequently, at present, due to the difficulty of the prevention of the reaction, the unsaturated vapor pressure type high pressure sodium lamp is still not made fit for practical use.